1. Field of the Invention
This invention relates to a roller bearing and in particular, though not exclusively, to a roller bearing for use in applications, such as in the gear units of wind turbines, in which skidding (also known as sliding) may occur between the rollers and raceway (bearing) surfaces.
2. Description of Related Art
When the rotational speed of rollers in a roller bearing is insufficient to ensure pure rolling of the raceways, skidding may occur between the rollers and raceway surfaces. Rollers typically decelerate in their rotational speed when in the unloaded zone of the bearing and accelerate in rotational speed due to contact with the raceways when in the loaded zone. The force needed to accelerate the rollers is, among other factors, dependent on the speed difference between the roller and raceway surfaces, and also the roller inertia. Prolonged skidding is undesirable because it can cause surface damage, such as material smearing, and lead to bearing failure. In particular, metal to metal contact can occur when the aforementioned operating conditions lead to an insufficient oil layer thickness, that resulting in a generation of heat which can cause skid smearing and subsequent bearing failure.
When bearings run under low or zero load conditions, the loaded zone becomes very small or does not exist. In consequence relative sliding between the roller and raceway surfaces is likely to occur. If a load is then suddenly applied to the bearing and the relative surface speeds of the rollers and raceways is high, smearing of material could occur as the rollers are accelerated. Over a period of time this can lead to significant surface damage.
In the multi-stage gear units of wind turbines the aforementioned conditions often can arise in respect of the bearings of the high speed and intermediate shafts. Turbulence in the wind can cause drastic load changes on the gear unit and cause rollers to decelerate during low-load and accelerate again when the load increases. During controlled deceleration of the wind turbine rotor blades, e.g., to a stationary condition, the torque direction on the shafts of the gear unit can often reverse, leading to a shift in the loaded zone position and roller skidding.